Metals from Groups I and II of the periodic table are commonly used to initiate the polymerization of monomers into polymers. For example, lithium, barium, magnesium, sodium, and potassium are metals that are frequently utilized in such polymerizations. Initiator systems of this type are of commercial importance because they can be used to produce stereo regulated polymers. For instance, lithium initiators can be utilized to initiate the anionic polymerization of isoprene into cis-1,4-polyisoprene or to initiate the polymerization of 1,3-butadiene into high vinyl polybutadiene.
The polymers formed in such polymerizations are terminated with the metal used to initiate the polymerization and are sometimes referred to as living polymers. They are referred to as living polymers because the polymer chains which are terminated with the metal initiator continue to grow or live until all of the available monomer is exhausted. Polymers that are prepared by utilizing such metal initiators normally have structures which are essentially linear and normally do not contain appreciable amounts of branching.
Such polymers are normally vulcanized or cured utilizing agents, such as sulfur, which promote crosslinking between the backbones of polymer chains. This causes such polymers to contain large concentrations of dangling chain end segments. It has been established that within a series of polymers of the same composition that tan delta at 60.degree. C. decreases with increasing polymer molecular weight (assuming the same degree of branching). This reduction in tan delta at 60.degree. C. with increasing molecular weight is a direct result of a lower concentration of dangling chain end segments in the cured rubber. Therefore, the trend in tire polymer synthesis is toward very high molecular weight elastomers. This is because a low tan delta at 60.degree. C. is indicative of low hysteresis and consequently tires made utilizing such elastomers with low tan delta values have lower rolling resistance.
The trend toward reducing tan delta at 60.degree. C. by producing higher molecular weight elastomers has introduced problems in both polymer synthesis and processing. For instance, increasing the molecular weight of polymers made utilizing organolithium catalysts requires lowering the initiator concentration which results in lower production rates. It is also much more difficult to process and mix elastomers having very high molecular weights in the compounding stage.